Add nested TAIT inference test

Author: spastorino

src/test/ui/type-alias-impl-trait/nested-tait-inference.rs

@@ -0,0 +1,18 @@
+// check-pass
+
+#![feature(type_alias_impl_trait)]
+#![allow(dead_code)]
+
+use std::fmt::Debug;
+
+type FooX = impl Debug;
+
+trait Foo<A> { }
+
+impl Foo<()> for () { }
+
+fn foo() -> impl Foo<FooX> {
+    ()
+}
+
+fn main() { }